Please remember that any mention of competitors, hinting at competitors or offering to provide details of competitors will result in an account suspension. The full rules can be found under the 'Terms and Rules' link in the bottom right corner of your screen. Just don't mention competitors in any way, shape or form and you'll be OK.
I would assume the GTX 780 would be priced at £600 like the 590's were.
I get what you mean, though I hope there's more of a difference than there is between a 480 and 580 (5 - 15%).